It was already known that clear sky up to several kilometres away
from clouds appears brighter than cloud-free sky elsewhere. People had
assumed that this was due to varying levels of tiny particles in the
air — now research shows that the effect is actually mostly due to
light reflected off the cloud and bouncing off the particles.

This
seemingly innocuous finding could have a surprisingly big knock-on
effect because it means there may be fewer cooling particles in the sky
than previously thought. And that could change the way we model climate
change.

To discover why the air near clouds appears so aglow, Tamas Varnai and Alexander Marshak at the NASA Goddard Space Flight Center in Greenbelt, Maryland, used MODIS satellite observations from a piece of sky above the Atlantic just southeast of the UK.

Previous
studies had speculated that whilst some of the glare may be due merely
to measurement artefacts, it might also be the case that more of the
small airborne particles aggregate around clouds or swell in size as
they absorb some of the cloud's water.

"Aerosols
measure only about a tenth of a micron, which is really, really tiny
compared to cloud droplets, which are often roughly 10 microns across,"
says Marshak.

Because
of their small size, the sky appears clear even where aerosol particles
abound. To work out the amount of particles suspended in the air,
Marshak's team looked at the number of clear sky pixels picked up by
MODIS and subtracted the reflection that was estimated to come from the
planet's surface and air molecules.

"This
leaves us with the remaining reflection bouncing off aerosol particles,
and so we can estimate their density," explains Marshak.

Using
this idea, it makes sense to assume that where the sky appears
brighter, light must be being reflected off more or bigger aerosol
particles — unless there is more light in the first place, of course.

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Skewed models

In
their current analysis, Marshak and Varnai found that the bright sky
effect was stronger on the sunlit sides of clouds or when the clouds
were denser.

Because
more light reflects off a denser or sunlit cloud, this suggests that
the clear sky brightness near clouds is caused by extra light
reflecting off the clouds sideways and then scattering again between
the particles in the clear sky area before reaching the satellite.
"It's essentially extra energy bouncing off the clouds that enhances
the glow of the clear sky," he says.

This
effect — called 3D radiative interaction — had been previously
identified as a factor cranking up the sky's brightness, but the new
data elevates it to the most important factor. This, in turn, means
that many estimates of aerosol density may be plain wrong, because most
clear sky analyses are close enough to clouds to be affected by the
effect, says Marshak.